Expressing nth order char-oxygen reaction initial rate under elevated pressure

Char combustion is investigated in the temperature range of 559-1273 K and pressure range of 0.1-1.6 MPa in a 0.5th-order reaction (a lignite coal char) and a first-order reaction (a bituminous coat char). The intrinsic combustion constant k(v) and the reaction order n do not vary with pressure, but the char combustion initial rate varies with the pressure in the chemical kinetics control and the internal diffusion influence regimes. The variation of initial rate with pressure was also sensitized to the reaction order n. In the chemical kinetics control regime, the char combustion initial rate was proportional to P-n with the exponent n, the reaction order. In the internal diffusion influence regime, the apparent dependency of char combustion initial rate on pressure changed with the exponent range from n to n/2, The reaction initial rate in the external diffusion control regime was invariant with pressure. A rate expression was developed for predicting the initial rate of nth-order char combustion under elevated pressure.